The web ui has upgrade its web components dependencies to latest versions.
This can impact web-ui plugin.

Web server does not provide /png and /redirect anymore (bug #3357).
This functionality is used to implement build status images.
This should be easy to implement if you need it.
One could port the old image generation code, or implement a redirection to http://shields.io/.

Support of worker-side usePTY was removed from buildbot-worker.
usePTY argument was removed from WorkerForBuilder and Worker classes.

html is no longer permitted in 'label' attributes of forcescheduler parameters.

Collapse Request Functions now takes master as first argument.
The previous callable contained too few data in order to be really usable.
As collapseRequests has never been released outside of beta, backward compatibility with previous release has not been implemented.

This is the last version of buildbot nine which supports python 2.6 for the master.
Next version will drop python 2.6 support.